1. Field of the Invention
The present invention relates to a pattern writing apparatus and to a method for converting writing data. For example, the present invention relates to a writing apparatus which writes a predetermined pattern on a target workpiece by using an electron beam, and to a method for converting writing data to be processed in the writing apparatus.
2. Description of Related Art
The lithography technique that advances microscale semiconductor devices is extremely important as being the only process to form patterns in semiconductor manufacturing processes. In recent years, with high integration of large-scale integrated circuits (LSI), critical dimensions required for semiconductor device circuits are shrinking year by year. In order to form a desired wiring pattern on semiconductor devices, a master pattern (called a mask or a reticle) of high precision is required. The electron beam writing technique intrinsically having excellent resolution is used for producing such highly precise master patterns.
FIG. 11 is a schematic diagram illustrating operations of a variable-shaped electron beam (EB) type pattern writing apparatus. As shown in the figure, the variable-shaped electron beam writing apparatus, including two aperture plates, operates as follows: A first aperture plate 410 has a rectangular opening or “hole” 411 for shaping an electron beam 330. This shape of the rectangular opening may also be a square, a rhombus, a rhomboid, etc. A second aperture plate 420 has a variable-shaped opening 421 for shaping the electron beam 330 that passed through the opening 411 into a desired rectangular shape. The electron beam 330 emitted from a charged particle source 430 and having passed through the opening 411 is deflected by a deflector to pass through a part of the variable-shaped opening 421 and thereby to irradiate a target workpiece or “sample” 340 mounted on a stage which continuously moves in one predetermined direction (e.g. X direction) during the writing or “drawing.” In other words, a rectangular shape formed as a result of passing through both the opening 411 and the variable-shaped opening 421 is written in the writing region of the target workpiece 340 on the stage. This method of forming a given shape by letting beams pass through both the opening 411 and the variable-shaped opening 421 is referred to as a “variable shaped” method.
When performing the electron beam writing, layout of a semiconductor integrated circuit is first designed, and then, layout data (design data), in which pattern layout is defined, is generated. Then, the layout data is converted into writing data which is adapted to the electron beam writing apparatus. The writing data is input into the writing apparatus, and, after plural data processing operations, generated as shot data to be used at the time of writing (refer to e.g., Japanese Patent Application Laid-open (JP-A) No. 2007-128933). Writing processing is performed based on the shot data. In the pattern writing apparatus, first, the writing data is developed into intermediate data prior to the shot data being generated. Conventionally, the pattern data format has been designed to respond to all possible sizes, coordinates, figure types, and the number of figures. Therefore, in the conventional pattern data format, the number of bits capable of responding to any of these is prepared.
However, depending on the layout of writing data, there is a case of using only a part of the number of bits prepared in the conventional pattern data format. For example, in the case of the layout where patterns of the same shape or the same size are mainly repeatedly used, only a few numbers of bits prepared in the conventional pattern data format are used. When only several patterns in one layout have such a case of only a few numbers of bits being used, they do not have much influence. However, with the recent trend of pattern miniaturization and pattern number increase, patterns using only a few numbers of bits are increasing. Therefore, if the numbers of bits which are not used are accumulated, it will become the number of bits corresponding to a data size not to be disregarded for the throughput of the apparatus.
As mentioned above, in the pattern data format conventionally used, the number of bits capable of responding to all possible sizes, coordinates, figure types, and the number of figures is prepared. Therefore, there are many unused bits, so that if the numbers of the unused bits are accumulated, it corresponds to a data size not to be disregarded for the throughput of the apparatus. As reducing the data size is requested with the recent trend of pattern miniaturization and pattern number increase, it is an issue how to reduce the number of bits used in the data.